With the population growth in large cities and the consequent increase in residential units located in apartment buildings, it is thus fundamental the use of indoor antennas for the reception of the Digital TV signal in the television sets, in view of the difficulty in installing an outdoor antenna in the buildings and condominiums, or in having collective distribution systems of good quality. The signal received from the television broadcasts may have vertical, horizontal, circular or elliptical polarization, according to the polarization used in the broadcaster antenna, or due to degradation caused to the waves by the multiple reflections of the signal in several obstacles, such as buildings, hills, etc.
Another feature of the antennas commonly used, either indoor or outdoor, consists in the fact that they were designed for receiving analog TV, and are expected to be able to receive signals in all the corresponding frequency ranges, that is, from 54 MHz (channel 2) up to 806 MHz (channel 69).
In Brazil, however, the digital TV uses only the frequency ranges from 174 MHz (channel 7). Thus, the conventional antennas will supply a digital receiver with unneeded signals, which may cause intermodulation and other types of interferences. Particularly, it is known that radio broadcasting signals in Frequency Modulated (FM), which occupy the frequency range from 88 MHz to 108 MHz, significantly impair the digital TV reception in most of the devices commercialized in the country.
Other signals that may potentially interfere in the reception, besides the FM broadcasters and the analog TV channels (channels 2 to 6), comprise those signals related to the communication systems used by the Firefighters, Police, Air Traffic, etc. These services use frequency ranges situated between 108 and 174 MHz.
A known and usual antenna construction for reception of the analog or digital TV signal in indoor environments comprises the provision of a dipole having one fourth of the wave length of the signal to be received, as schematically illustrated in FIG. 1 of the attached drawings. This known antenna construction includes a base 1, a support structure 2 and two irradiating elements 3, whose lengths may be adjusted according to the channel being tuned. The coaxial cable 4 feeding the television set receiver has the outer mesh thereof connected to one of the irradiating elements 3, and the central conductor of the coaxial cable 4 is connected to the other irradiating element 3.
In the type of prior art construction illustrated in FIG. 1, the antenna receives the signals having a field polarization direction parallel to the irradiating elements 3. However, the conventional TV broadcasts may include the emission of waves having a vertical, circular or elliptical polarization. Thus, in case the antenna needs to receive the different types of signal coming from different transmitter stations, or even the reflected signals, the angle of reception of the antenna has to be adjusted for each type of polarization.
In the exemplary construction illustrated in FIG. 1, the antenna may receive, clearly, the horizontally polarized signals. However, the antenna is not able to simultaneously receive signals having polarizations other than the horizontal. In this case, in which it is only possible an adequate reception of horizontally polarized signals, all the receptions have to be mandatorily made horizontally. These antennas do not have means to allow the user to receive signals in multiple and different polarizations.
Besides the operational limitation mentioned above, this known type of antenna has a dimension and shape which makes difficult the aesthetic harmonization with the indoor environment in which they are installed. Another drawback of these known antennas is due to the fact that they are not suitable to be installed outdoors or in vehicles.
Another important factor that degrades the digital TV signal reception is the interference caused by the FM radio transmitters. These hi-power analog signals, irradiated by the transmission antennas of the FM stations scatter throughout the spectrum and interfere in the reception of the digital TV signals. This reason alone is sufficient to disqualify said known indoor antennas, once they do not have filters to eliminate said problem.
Aiming to eliminate the limitations related to signal reception and also to outdoor or vehicular installation of the known antennas mentioned above, it was developed the antenna object of the Brazilian patent application PI0904735-2, which discloses a low cost, reduced size and easily installable antenna construction, and which is adjustable to provide a simultaneous reception of signals transmitted with horizontal, vertical, circular or elliptical polarizations, that is, an omnidirectional antenna construction which allows for an adequate reception in the full bandwidth of the digital TV, for example, the range from 174 MHz to 806 MHz.
Said second antenna construction provides a base 1, made of metal and superiorly carrying a hinge 5 in which is mounted an irradiating element 3, in the form of a metallic rod and which may be displaced between operational positions, in which it forms an angle between zero and 90° in relation to the plane of the base. The coaxial cable C connected to the receiver device of the TV signals has its mesh M electrically connected to the base 1, and its central wire F connected to the irradiating element 3 through said hinge 5. This prior construction further comprises an annular permanent magnet 6 mounted on the base 1 and inside a housing 7.
Although eliminating some of the disadvantages of the previous antennas, said last construction still presents a few drawbacks, including a relevant signal loss through the hinge, especially over time and with a progressive deterioration of the elements articulated to each other.
Moreover, said previous construction is relatively complex, using multiple elements for building the base and for promoting the connection of the elements of the coaxial cable to the metallic base and to the irradiating element through the hinge, adding cost to the product and impairing the integral transmission of the incoming signals to the coaxial cable connected to the receiver device.
Still another drawback of said prior solution is related to the fact that it does not include any filter element provided between the irradiating element and the coaxial cable, in order to block de radio broadcast signals in modulated frequency occupying the frequency range from 88 MHz to 108 MHz, which, as mentioned before, significantly impair the reception of the digital TV signals in most part of the devices sold in Brazil. The filters provided inside the receiver device itself are not able to prevent undesirable interferences of said radio broadcast signals in the reception of the digital TV signals.